Dr. James Knoll, a forensic psychiatrist, offered a psychological perspective on the case, and answered viewers' questions about how someone could commit these kinds of crimes.

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Knoll is the director of forensic psychiatry and associate professor of psychiatry at SUNY Upstate Medical University in Syracuse, N.Y. He has worked as a forensic evaluator for state and federal courts, corrections and the private sector. Click here to visit Knoll's blog.

SARA: Does Stacey Castor's behavior and body language on the stand show any sort of clue about her mental state and guilt?

KNOLL: I think it is only natural to want to look for signs like this, especially when the defendant may have done something very cruel or terrible. However, body language is not a science that one can rely on with scientific certainty. Further, one must remember that there is much going on during a trial that can affect a defendant's behavior and body language. Examples include side effects of medications, anxiety, fatigue and attorney instructions to the defendant on how to behave, to name just a few.

Black Widow: The Mystery Begins

But your question raises a salient emotional issue: Our own anxiety about being able to recognize or "spot" a murderer. The reality is that the face, behavior and body language of a murderer are very often rather ordinary and provide no prospective cues for recognition. From Eichmann to a long list of serial killers, the phrase "terrifyingly normal" is sometimes used. The absence of a "look" or a mark of Cain is highly disturbing to most. A journalist covering the Jeffrey Dahmer trial "could not get over" how ordinary Dahmer appeared, remarking that "there was nothing to him."
[i] Schwartz A: The man who could not kill enough. Birch Lane, N. Y., 1993.

CHRIS, Rochester, N.Y.: I noticed during Stacey Castor's testimony a peculiar thing. When asked by the prosecutor whether or not she killed either of her husbands, she replied no and absolutely not. Oddly though, she was shaking her head up and down as if subconsciously saying yes. Is this a nonverbal communication that is valid or just coincidence?

KNOLL: It is tempting to want to read subtleties into details such as this, and you are obviously a very perceptive person. However, as mentioned previously, this really amounts to speculation, with no real reputable science to back it up. For example, it is equally possible that she was nodding her head affirmatively simply to put emphasis behind her answers.

LAURA, Dunfermline, Ill.: I'm wondering if Stacey is a psychopath and what determines if a psychopath kills or just makes other people's lives miserable who are subject to them?

KNOLL: Being a psychopath may make someone more likely to murder than a nonpsychopath, but to the best of our current and limited knowledge, what leads people to commit acts of intentional harm involves a complex interaction of biological, psychological and social factors acting in concert with situational variables. One set of factors affects and is affected by the others, and likely cannot stand on its own. In any individual case, the forensic psychiatrist must objectively weave them together in an accurate, coherent narrative.